MicroRNA-145 attenuates IL-6-induced enhancements of sensitivity to UVB irradiation by suppressing MyD88 in HaCaT cells.

MicroRNAs (miRNAs/miRs) play vital roles in various immune diseases including systemic lupus erythematosus (SLE). The current study aimed to assess the role of miR-145 in interleukin-6 (IL-6)-treated HaCaT cells under ultraviolet B (UVB) irradiation and further explore the potential regulatory mechanism. HaCaT cells were pretreated with IL-6 and then exposed to UVB to assess the effect of IL-6 on sensitivity of HaCaT cells to UVB irradiation. The levels of miR-145 and MyD88 were altered by transfection and the transfected efficiency was verified by quantitative reverse transcription polymerase chain reaction (qRT-PCR)/western blot analysis. Cell viability, percentage of apoptotic cells and expression levels of apoptosis-related factors were measured by trypan blue assay, flow cytometry assay, and western blot analysis, respectively. In addition, the levels of c-Jun N-terminal kinases (JNK) and nuclear factor-κB (NF-κB) signaling pathway-related factors were assessed by western blot analysis. IL-6 treatments significantly aggravated the reduction of cell viability and promotion of cell apoptosis caused by UVB irradiation in HaCaT cells. Interestingly, miR-145 level was augmented by UVB exposure and miR-145 mimic alleviated IL-6-induced increase of sensitivity to UVB irradiation in HaCaT cells, as dramatically increased cell viability and reduced cell apoptosis. Opposite effects were observed in miR-145 inhibitor-transfected cells. Meanwhile, MyD88 was negatively regulated by miR-145 and MyD88 mediated the regulatory effect of miR-145 on IL-6- and UVB-treated cells. In addition, miR-145 mimic inhibited the JNK and NF-κB pathways by down-regulating MyD88. In conclusion, the present study demonstrated that miR-145 alleviated IL-6-induced increase of sensitivity to UVB irradiation by down-regulating MyD88 in HaCaT cells.

Introduction

Systemic lupus erythematosus (SLE) is characterized by the generation of autoantibodies and high levels of immune complexes precipitation,[1] which might induce damages of tissues or organs of whole body, especially kidneys.[2] The SLE frequently occurs in females with reproductive age, which accounts for 90% SLE patients.[3] There are more than 80% of patients with SLE manifesting clinical presentations of skin lesions, multiform erythema and diverse rashes, and the cutaneous lesions have been indicated as one of the most prominent clinical features of SLE.[4] Ultraviolet B (UVB) irradiation could exacerbate the process of SLE through induction of DNA damages, inflammatory responses, and dysfunction of keratinocytes.[5] Among them, the inflammatory responses of keratinocytes play a crucial role in the skin lesions of SLE. Therefore, it is of great significance to explore the mechanism of inflammatory injury induced by UVB exposure in keratinocytes for the treatment of SLE.

MicroRNAs (miRNAs/miRs) are small and endogenous non-coding RNAs with length in 19–24 nucleotides, which have been reported to function as tumor suppressors or oncogenes in various cancers.[6-8] It has been widely approved that miRNAs play a critical role in the process of tumor development including apoptosis, migration, and proliferation through its regulatory role in gene expression at post-transcriptional levels.[9] miRNAs can cause inhibition of mRNA translation or induction of degradation through directly binding to the 3′ untranslated regions (3′-UTR) of targeted mRNAs.[10] Several miRNAs have been reported to be dysregulated in human patients with SLE, such as miR-101,[11] miR-148a,[12] miR-31,[13] and miR-155[14].[15] miR-145 has been emerged as a tumor suppressor in many kinds of tumors. For instance, Khan et al.[16] demonstrated that miR-145 overexpression suppressed cell growth and metastasis, as well as enhanced sensitivity to gemcitabine through targeting mucin 13 (MUC13) in pancreatic cancer cell lines. In addition, miR-145 has been reported to be abnormally expressed in T cells from SLE patients compared with normal healthy patients,[17] suggesting that miR-145 may be associated with the process of SLE. However, the exact role and potential mechanism of miR-145 in UVB irradiation-induced inflammatory injury have not been fully elucidated yet.

Interleukin-6 (IL-6) is a pleiotropic cytokine that is pivotal for inflammatory response.[18] A previous study has reported that IL-6 is an important factor implicated in the regulation of SLE.[19] In addition, IL-6 level was shown to be increased in cells treated by UVB irradiation.[20] Therefore, we hypothesized that IL-6 might affect the sensitivity to UVB irradiation. The present study aimed to assess the role of miR-145 in UVB-exposed and IL-6-treated keratinocyte cells and further explore the underlying mechanism. We found that the pretreatment of IL-6 significantly enhanced the sensitivity of HaCaT cells to UVB irradiation. Interestingly, the expression of miR-145 was significantly up-regulated by UVB exposure in HaCaT cells and miR-145 mimic attenuated the increase of sensitivity to UVB irradiation induced by IL-6 through down-regulation of myeloid differentiation primary response protein 88 (MyD88). In addition, we also found that the c-Jun N-terminal kinases (JNK) and nuclear factor-κB (NF-κB) signaling pathways were inhibited by miR-145 overexpression through down-regulation of MyD88. These results might provide a novel therapeutic target for the treatment of SLE.

Materials and methods

Cell culture and treatment

Human keratinocytes (HaCaT cells) were obtained from Cell Lines Service (CLS; Eppelheim, Germany). Cells were maintained in Dulbecco’s modified Eagle’s medium (DMEM; Gibco BRL, Carlsbad, CA, USA), which was supplemented with 10% heated-inactivated fetal bovine serum (FBS; Gibco BRL), 100 U/mL penicillin G, and 100 mg/L streptomycin (both from Amresco, Solon, OH, USA), and cells were cultured in a humidified cell culture incubator containing 5% CO2 and 95% air at 37°C. For the treatment of IL-6, HaCaT cells were incubated in DMEM containing IL-6 (Sigma-Aldrich, St. Louis, MO, USA) for 2 h.

UVB irradiation

For the treatment UVB irradiation, HaCaT cells were seeded on the culture plates and incubated at 37°C in a humidified cell culture incubator containing 5% CO2 and 95% air until reached to about 90% confluence. Before irradiation, cells were pre-treated with or without IL-6 (20 ng/mL) for 2 h. Then, the culture medium containing IL-6 was replaced with sterile phosphate buffered saline (PBS) and the cells were exposed to UVB irradiation at 280–320 nm using a UVB lamp supplied by Spectronics Corp. (Westbury, NY, USA) at a fluence rate of 30 mJ/cm2. After UVB irradiation, the cells were maintained with fresh culture medium and were used for the subsequent experiments.

Cell transfection

For miRNA transfection, miR-145 mimic, miR-145 inhibitor, and the negative control (Scramble) were synthesized by GenePharma Co. (Shanghai, China) and were applied to alter the expression of miR-145 in HaCaT cells. HaCaT cells were transfected with those vectors through using Lipofectamine 3000 reagent (Life Technologies Corporation, Carlsbad, CA, USA) following manufacturer’s instructions and incubated for 48 h. Then, the transfected cells were generated and used for the following experiments.

For the overexpression of MyD88, the full-length MyD88 sequences were ligated into empty pcDNA3.1 vector (GenePharma) to form the MyD88-overexpressing vectors and were referred to as pc-MyD88. The empty pcDNA3.1 was served as a negative control of pc-MyD88. Then, HaCaT cells were transfected with either pc-MyD88 or empty pcDNA3.1 through using Lipofectamine 3000 reagent (Life Technologies Corporation) on the basis of manufacturer’s protocol. After transfection for 48 h, cells were harvested and employed for subsequent experiments.

Quantitative reverse transcription polymerase chain reaction

The expression of miR-145 was measured by quantitative reverse transcription polymerase chain reaction (qRT-PCR) analysis. Briefly, total RNAs were extracted from cells with the use of TRIzol reagent (Invitrogen, Carlsbad, CA, USA). Then, RNA was reverse transcribed to cDNA with the use of the Multiscribe RT kit (Applied Biosystems, Foster City, CA, USA), and real-time PCR was performed using Taqman Universal Master Mix II (Applied Biosystems) following the manufacturer’s protocols. The expression of miR-145 was normalized to U6 using the 2−ΔΔCt method as previously reported.[21]

Cell viability assay

Trypan blue exclusion was applied to determine the cell viability of HaCaT cells. In brief, cells were plated in 24-wells plates at a density of 1 × 105 cells/well. After the treatment of IL-6 and UVB irradiation, the cells were rinsed with PBS and the living cell numbers were determined by staining with 0.4% solution of trypan blue (Beyotime Biotech, Shanghai, China) as previous described.[22]

Apoptosis assay

The apoptotic cells were determined by flow cytometry analysis using an Annexin V-FITC/PI apoptosis detection kit (Beijing Biosea Biotechnology, Beijing, China). Briefly, HaCaT cells at a concentration of 1 × 105 cells/well were seeded in six-well plates and were incubated at 37°C in a humidified atmosphere of 5% CO2 and 95% air. After treatments of IL-6 and UVB irradiation, cells were generated and washed with cold PBS for three times. Then, cells were resuspended in binding buffer, followed by staining with 5 μL Annexin V-FITC and 10 μL PI solution for 30 min in the dark at room temperature. Then, the apoptotic cells were differentiated by flow cytometer (Beckman Coulter, Fullerton, CA, USA) and quantified by FlowJo software (Tree Star, Inc., Ashland, OR, USA).

Western blot analysis

Briefly, cells were harvested and washed with cold PBS for three times, and then the cells were homogenized in radio-immunoprecipitation assay (RIPA) lysis buffer supplemented with protease inhibitor cocktails (both form Jiancheng, Nanjing, China) to extract protein. The concentration of protein samples was quantified by the BCA™ Protein Assay Kit (Beyotime Biotech). Protein samples (30 μg) were electrophoresed on sodium dodecyl sulfate–polyacrylamide gels and blotted onto polyvinylidene difluoride (PVDF) membranes (Millipore, Bed-ford, MA, USA). Then, the membranes were blocked with 5% bovine serum albumin (BSA; Roche, Basel, Switzerland) for 1 h at room temperature and incubated overnight at 4°C with specific primary antibodies against Bax (#5023), Bcl-2 (#4223), caspase-3 (#9665), caspase-9 (#9502), MyD88 (#4283), JNK (#9252), p-JNK (#4668), c-jun (#9165), p-c-jun (#3270), p65 (#8242), p-p65 (#3033), IκBα (#4812), p-IκBα (#2859), and β-actin (#4970; Cell Signaling Technology, Beverly, MA, USA). The dilution of primary antibodies was 1: 1000 in 5% BSA. Then, the membranes were washed with Tris-buffered saline plus Tween-20 (TBST), followed by incubation with secondary antibody marked by horseradish peroxidase for 1 h at room temperature. The blots were detected with enhanced chemiluminescence (ECL) reagent kit (Vazyme Biotech, Nanjing, China) and developed with Bio-rad gel imaging system (Bio-Rad, Hercules, CA, USA). The intensity of the protein bands was analyzed by Image Lab™ software (Bio-Rad).

Statistical analysis

Experiments were performed in triplicate with three repeats. We evaluated the statistical significance with the performance of the one-way analysis of variance (ANOVA) test followed by Dunnett’s post hoc test, using GraphPad Prism 6 software (GraphPad Software, San Diego, CA, USA). All results are presented as the mean ± standard deviation (SD) of three independent experiments. The value of P < 0.05 was considered to be significant.

Results

IL-6 treatment enhanced the sensitivity of HaCaT cells to UVB irradiation

HaCaT cells were exposed to a series concentration of IL-6 (10–40 ng/mL) for 2 h and then cell viability was measured. Results in Figure 1(a) showed that when the dosage of IL-6 was not more than 20 ng/mL, there was no significant difference in viability of HaCaT cells (P > 0.05). However, a significant reduction of cell viability was observed in IL-6-treated HaCaT cells when the concentrations of IL-6 were 30 and 40 ng/mL (P < 0.05 or P < 0.01). Thus, 20 ng/mL was selected as the treated dosage of IL-6 in the subsequent experiments. Then, we explored the combinative treatment of IL-6 exposure and UVB irradiation. As shown in Figure 1(b), the cell viability was significantly decreased in UVB-treated HaCaT cells and IL-6 + UVB-treated cells relative to the PBS group (P < 0.01 or P < 0.001). Interestingly, the combinative treatment of IL-6 exposure and UVB irradiation induced a significant reduction of cell viability, compared with the UVB irradiation alone (P < 0.01). Similar observations were found in cell apoptosis, showing that UVB irradiation significantly increased apoptotic cell rates (Figure 1(c) and Supplementary Figure S1; P < 0.001), enhanced the expression levels of pro-apoptotic factors (Bax, cleaved-caspase-3, and cleaved-caspase-9), and inhibited the anti-apoptotic Bcl-2 expression levels (Figure 1(d)). In addition, the combination of IL-6 and UVB treatments dramatically aggravated the cell apoptosis induced by UVB irradiation, for the fact that a more significant induction of apoptotic cells and more visible changed trend of apoptosis-related factors were observed in the IL-6 + UVB group (Figure 1(c) and (d) and Supplementary Figure S1; P < 0.01 or P < 0.001). Overall, it appeared that IL-6 exposure enhanced the sensitivity of HaCaT cells to UVB irradiation.

Figure 1.

IL-6 treatment enhanced the sensitivity of HaCaT cells to UVB irradiation. HaCaT cells were incubated with IL-6 (10, 20, 30, and 40 ng/mL) for 2 h. (a) Cell viability was measured by trypan blue exclusion. HaCaT cells were pretreated with IL-6 (20 ng/mL) for 2 h and then exposed to UVB irradiation for 6 h. Then, (b) cell viability, (c) percentage of apoptotic cells, and (d) the protein expression of apoptosis-related key factors were respectively determined by trypan blue assay, flow cytometry assay, and western blot analysis. *P < 0.05, **P < 0.01, ***P < 0.001.

Figure S1

Representative dot plots of flow cytometry in Figure 1B. HaCaT cells were pretreated with IL-6 (20 ng/mL) for 2 h and then exposed to UVB irradiation for 6 h. Apoptotic cells were quantified by flow cytometry assay.

UVB irradiation up-regulated the expression of miR-145 in HaCaT cells

HaCaT cells were exposed to UVB for 3, 6, and 12 h, and then the expression of miR-145 was detected. As shown in Figure 2(a), qRT-PCR analysis indicated that the expression of miR-145 was dramatically up-regulated by UVB exposure and reached to the peak when the irradiated time was 6 h (P < 0.05, P < 0.01, or P < 0.001).

Figure 2.

The expression of miR-145 was up-regulated by UVB irradiation and was altered by transfection. HaCaT cells were treated with UVB irradiation for 0, 3, 6, and 12 h. (a) The expression of miR-145 was measured by qRT-PCR. HaCaT cells were transfected with miR-145 inhibitor, miR-145 mimic, or scramble for 48 h. Then, (b) qRT-PCR was performed to verify the transfected efficiency. *P < 0.05, **P < 0.01, ***P < 0.001.

miR-145 mediated the synergistic effect of IL-6 treatment with UVB irradiation on HaCaT cells

We further explored whether miR-145 was associated with the effect of IL-6 treatment on UVB-irradiated HaCaT cells. HaCaT cells were transfected with miR-145 inhibitor or miR-145 mimic to alter the expression of miR-145, and the transfected efficiency was identified by qRT-PCR analysis. As shown in Figure 2(b), the expression of miR-145 was obviously augmented in miR-145 mimic-transfected cells but down-regulated in miR-145 inhibitor-transfected cells relative to the scramble miRNA-transfected cells (P < 0.05 or P < 0.001). Then, the transfected cells or non-transfected cells were exposed to UVB irradiation or UVB + IL-6 treatments, and cell viability and cell apoptosis were assessed. Results in Figure 3(a) demonstrated that viability of HaCaT cells was dramatically increased by miR-145 overexpression while was significantly reduced by miR-145 inhibitor, as compared to the IL-6 + UVB + scramble group (P < 0.05 or P < 0.01). Flow cytometry analysis and western blot results revealed that overexpression of miR-145 significantly attenuated the IL-6-induced increase of apoptosis in UVB-irradiated cell, as reduced the apoptotic cell rates (Figure 3(b) and Supplementary Figure S2, P < 0.05), promoted Bcl-2 expression, and inhibited the levels of pro-apoptotic factors including Bax, cleaved-caspase-3, and cleaved caspase-9 (Figure 3(c)). Opposite results were observed in the cells transfected with miR-145 inhibitor, showing that miR-145 silence significantly increased the number of apoptotic cells (Figure 3(b) and Supplementary Figure S2; P < 0.05), and enhanced the expression of factors related with pro-apoptosis but inhibited the anti-apoptotic factor expression (Figure 3(c). These results suggested that miR-145 overexpression attenuated the IL-6-induced increase of sensitivity to UVB irradiation, while miR-145 silence aggravated the sensitivity to UVB irradiation.

Figure 3.

miR-145 mediated the synergistic effect of IL-6 treatment with UVB irradiation on HaCaT cells. HaCaT cells were transfected with miR-145 inhibitor, miR-145 mimic, or scramble for 48 h. Then, the transfected or non-transfected cells were exposed to UVB irradiation alone or combination of IL-6 pretreatment and UVB irradiation. Thereafter, (a) cell viability, (b) percentage of apoptotic cells, and (c) protein expression of core factors related with apoptosis were respectively detected by trypan blue exclusion, flow cytometry assay, and western blot analysis. *P < 0.05, **P < 0.01, ***P < 0.001.

Figure S2

Representative dot plots of flow cytometry in Figure 3B. HaCaT cells were transfected with miR-145 inhibitor, miR-145 mimic, or Scramble for 48 h. Then, the transfected or non-transfected cells were exposed to UVB irradiation alone or combination of IL-6 pretreatment and UVB irradiation. Apoptotic cells were quantified by flow cytometry assay.

The expression of MyD88 was negatively regulated by miR-145 in HaCaT cells

It has been reported that toll-like receptors (TLRs) are implicated in the pathogenesis of autoimmune diseases including SLE.[23] MyD88 is the universal intracellular adaptor which located at downstream of TLRs.[24] Therefore, the interaction between miR-145 and MyD88 was studied subsequently. As shown in Figure 4(a), the protein expression of MyD88 was obviously down-regulated in miR-145 mimic-transfected cells, while was significantly up-regulated in miR-145 inhibitor-transfected cells relative to the scramble group (P < 0.05 or P < 0.001). These results indicated that miR-145 negatively regulated the expression levels of MyD88.

Figure 4.

The expression of MyD88 was negatively regulated by miR-145 and was overexpressed by transfection with pc-MyD88. HaCaT cells were transfected with miR-145 inhibitor, miR-145 mimic, or scramble for 48 h. (a) The expression of MyD88 was determined by western blot analysis and the relative band intensity was quantified by Image Lab™ software. HaCaT cells were transfected with pcDNA3.1 or pc-MyD88 for 48 h and (b) the efficiency of transfection was verified by western blot analysis. *P < 0.05, ***P < 0.001.

miR-145 overexpression attenuated the IL-6-induced increase of sensitivity to UVB irradiation through down-regulation of MyD88

We then investigated whether MyD88 was involved in the regulatory role of miR-145 in the sensitivity of IL-treated HaCaT cells to UVB irradiation. The expression level of MyD88 was forcedly altered by transfection with pc-MyD88 vector, and western blot analysis was performed to verify the transfected efficiency. As shown in Figure 4(b), the protein expression of MyD88 was efficiently augmented in HaCaT cells after transfection with pc-MyD88 vector relative to the pcDNA3.1-transfected cells (P < 0.001). We found that MyD88 overexpression dramatically declined the effects of miR-145 overexpression on cell viability of HaCaT cells under combinative treatment of IL-6 stimulation and UVB irradiation (Figure 5(a); P < 0.05). We also investigated the effect of MyD88 overexpression on cell apoptosis in IL-6- and UVB-treated HaCaT cells which were transfected with miR-145 mimic. We found that MyD88 overexpression abolished the effect of miR-145 overexpression on the sensitivity of IL-6-treated HaCaT cells to UVB irradiation, as MyD88 overexpression significantly elevated the apoptotic cell rates (Figure 5(b) and Supplementary Figure S3; P < 0.01), and enhanced the expression levels of pro-apoptotic factors but suppressed the Bcl-2 expression (Figure 5(c)). Taken together, these results demonstrated that miR-145 overexpression attenuated the IL-6-induced increase of sensitivity to UVB irradiation through down-regulation of MyD88 in HaCaT cells.

Figure 5.

miR-145 overexpression attenuated the IL-6-induced increase of sensitivity to UVB irradiation through down-regulation of MyD88 in HaCaT cells. HaCaT cells were transfected with miR-145 mimic or co-transfected with miR-145 mimic and pc-MyD88 for 48 h. Then, the transfected or non-transfected cells were exposed to IL-6 treatment and UVB irradiation. Subsequently, (a) cell viability, (b) percentage of apoptotic cells, and (c) protein expression of core factors related with apoptosis were respectively detected by trypan blue exclusion, flow cytometry assay, and western blot analysis. *P < 0.05, **P < 0.01, ***P < 0.001.

Figure S3

Representative dot plots of flow cytometry in Figure 5B. HaCaT cells were transfected with miR-145 mimic, or co-transfected with miR-145 mimic and pc-MyD88 for 48 h. Then, the transfected or non-transfected cells were exposed to IL-6 treatment and UVB irradiation. Apoptotic cells were quantified by flow cytometry assay.

miR-145 overexpression inhibited the JNK and NF-κB signaling pathways through down-regulation of MyD88

To further investigate the potential mechanism of miR-145 in HaCaT cells, the JNK and NF-κB pathways were assessed. As shown in Figure 6(a), we found that IL-6 plus UVB treatments significantly increased the phosphorylation of JNK and c-jun (P < 0.01 or P < 0.001), and miR-145 mimic inhibited the expression of p-JNK and p-c-jun (P < 0.05 or P < 0.01). However, MyD88 overexpression reversed the effect of miR-145 mimic, as dramatically augmented the expression of p-JNK and p-c-jun relative to the IL-6 + UVB + miR-145 mimic + pcDNA3.1 group (P < 0.05 or P < 0.01). Similar results were observed in the NF-κB pathways, as the reduction of p-p65 and p-IκBα induced by miR-145 mimic was significantly reversed by the overexpression of MyD88 (Figure 6(b); P < 0.05 or P < 0.01). These results revealed that miR-145 overexpression suppressed the JNK and NF-κB cascades through down-regulation of MyD88 in HaCaT cells.

Figure 6.

miR-145 overexpression inhibited the JNK and NF-κB signaling pathways through down-regulation of MyD88. HaCaT cells were transfected with miR-145 mimic or co-transfected with miR-145 mimic and pc-MyD88 for 48 h. Then, the transfected or non-transfected cells were exposed to IL-6 treatment and UVB irradiation. The protein levels of (a) JNK and (b) NF-κB signaling pathway-related factors were assessed by western blot analysis, and the relative band intensity was quantified by Image Lab™ software. *P < 0.05, **P < 0.01, ***P < 0.001.

Discussion

SLE is a prototypic autoimmune system disease which is characterized by inflammatory response-induced damages in multiple organs and tissues including skin.[25] IL-6, as an immunomodulatory pleiotropic cytokine, possesses a wide range of biological properties and plays a critical role in the regulation of cellular processes such as cell proliferation, apoptosis, and differentiation.[26] It has been reported that IL-6 levels were highly expressed in SLE patients’ sera and in lupus mouse models,[27] suggesting that IL-6 levels might be closely associated with SLE. UVB irradiation has been reported to induce inflammatory response in skin of SLE and exacerbate skin damages of SLE patients.[28,29] In the present study, we found that IL-6 at a dosage of 20 ng/mL did not significantly reduce viability of HaCaT cells. However, IL-6 pretreatment notably aggravated cell injuries caused by UVB irradiation in HaCaT cells.

Previous studies have revealed that UVB irradiation could induce the alteration of miRNAs expression in HaCaT cells, such as the up-regulation of miR-139-5p,[30] miR-1246,[31] miR-23a,[32] and miR-141.[33] Meanwhile, the aberrant expression levels of miRNAs are frequently closely linked with cellular processes including apoptosis and inflammatory responses in UVB-irradiated HaCaT cells through modulating downstream genes or signaling cascades. For example, Li et al.[31] reported that miR-1246 overexpression down-regulated the expression of RTKN2 through directly binding to its 3′-UTR, thus enhancing cell apoptosis induced by UVB irradiation in HaCaT cells. Meanwhile, it has also reported that miR-23a possessed its anti-apoptotic role in UVB-irradiated HaCaT cells through down-regulation of cyclobutane pyrimidine dimers (CPDs).[32] In the present study, we also found that the expression of miR-145 was significantly up-regulated in UVB-irradiated HaCaT cells. Thus, we hypothesized that miR-145 might be involved in the increase of sensitivity to UVB irradiation induced by IL-6 pretreatment in HaCaT cells. We found that miR-145 overexpression significantly attenuated IL-6-induced increase of sensitivity to UVB irradiation, and miR-145 inhibitor exhibited an opposite effect. Our results demonstrated an anti-apoptotic role of miR-145 in UVB-exposed HaCaT cells, which were consistent with the role of miR-145 in tumors, as a tumor suppressor.

MyD88 was originally isolated as a myeloid differentiation primary response gene that is rapidly differentiated from M1 myeloleukemic cells into macrophages by the stimulation of IL-6.[34] As a common adaptor, MyD88 could interact with interleukin-1receptor-associated kinase 1/4 (IRAK1/4) and TNF receptor-associated factor 6 (TRAF6) through its amino-terminal death domain and induce the activation of NF-κB, thus enhancing the expression of pro-inflammatory factors.[35,36] It has been reported that the expression of MyD88 was aberrantly expressed in the B lymphocytes of SLE patients,[37] implying that MyD88 might be involved in the development and process of SLE. As the critical role of MyD88 in inflammatory response, we further investigated whether MyD88 was regulated by miR-145 and was associated with the IL-6-induced increase of sensitivity to UVB irradiation in HaCaT cells. We found that the expression of MyD88 was negatively regulated by miR-145 expression, and MyD88 overexpression reversed the effect of miR-145 overexpression on UVB- and IL-6-treated HaCaT cells.

The activation of JNK/c-jun pathway has been reported to mediate the cell injury in UVB-irradiated HaCaT cells.[38,39] Meanwhile, Kent et al.[40] found that overexpression of miR-145 could inhibit the activation of JNK in colorectal cancer cell line HCT116. Thus, we further investigated the effect of miR-145 on the JNK/c-jun pathway in UVB- and IL-6-treated HaCaT cells. Being consistent with previous studies, we found that IL-6 pretreatment and UVB irradiation activated the JNK/c-jun pathway, and miR-145 overexpression reversed the effect of UVB and IL-6, as reduced the phosphorylation of JNK and c-jun. In addition, we also found that MyD88 mediated the inhibitory effect of miR-145 overexpression on the activation of the JNK/c-jun pathway, as MyD88 overexpression abolished the effect of miR-145 overexpression. Similarly, we found that miR-145 overexpression inhibited the activation of the NF-κB pathway through down-regulation of MyD88. NF-κB, as a widely expressed transcription factor, has been reported to be activated by UVB irradiation in HaCaT cells.[41] In addition, MyD88 has been reported to mediate the interleukin-1 receptor (IL-1R)- and TLR-induced activation of NF-Κb.[42] Our results were consistent with those previous reports.

In conclusion, our current study demonstrated that miR-145 overexpression attenuated the IL-6-induced increase of sensitivity to UVB irradiation in HaCaT cells. The underlying mechanism might be partially through its negative regulation of MyD88. Our present study might provide a better understanding of miR-145 in SLE and might also provide new insight for the therapeutic target of SLE.